A processing circuit or a processing system is evaluated by inputting a pulse outputted at random to a circuit at a subsequent stage. The pulse is in many cases outputted at random as above, and includes radiation, cosmic rays and noises (e.g. white noise, thermal noise and so on). These pulses are not actually used, but a pulse is generated spuriously, and a processing circuit or a processing system at a subsequent stage is evaluated using the pulse generated spuriously (hereinafter called “spurious pulse”).
Here, the spurious pulse will be described, taking for example a radiation spurious pulse used in place of radiation used in a nuclear medicine diagnostic apparatus such as a PET (Positron Emission Tomography) apparatus. A PET apparatus is constructed to reconstruct a sectional image of a patient only when a plurality of detectors simultaneously detect a pair of γ-rays generated by annihilation of positrons.
Specifically, a radioactive drug including a positron-emitting radionuclide is injected into the body of a patient, and detectors consisting of numerous detecting element (e.g. scintillator) groups detect pair annihilation γ-rays of 511 KeV released from the patient injected. And when two detectors detect γ-rays within a definite period of time, they are counted as one pair of annihilation γ-rays detected as a coincidence, and a pair annihilation generating point is determined to exist on a straight line linking the detector pair having detected them. Such coincidence information is accumulated and reconstruction is carried out to obtain a positron-emitting radionuclide distribution image (i.e. a sectional image).
The PET apparatus obtains the above sectional image by inputting a radiation pulse obtained with the radiation detector (pulse converted into light by the scintillators) to a pulse processing circuit at a subsequent stage, and further processing the inputted radiation pulse in an arithmetic processing circuit. Usually, in order to evaluate the pulse processing circuit of the PET apparatus, the actual radiation pulse obtained with the radiation detector may be inputted to the circuit at a subsequent stage. In that case, since a radioactive source such as 22Na point radiation source is used, it is necessary to carry out the operation in a radiation controlled area. However, the pulse processing circuit can be evaluated outside the radiation controlled area, without using the radioactive source and radiation detector, by inputting a radiation spurious pulse generated by a spurious pulse generator to the pulse processing circuit at the subsequent stage. The spurious pulse generator is used also for purposes including evaluation of a processing substrate having such a circuit at a subsequent stage mounted thereon, evaluation of software for the spurious pulse generator, debugging at a time of a fault occurrence with the processing substrate or software (in order to remove the fault), an inspection apparatus for periodical checking of the PET apparatus, and verification (daily quality assurance) of the PET apparatus.
A pulse shape generator is used as the spurious pulse generator. An ordinary pulse shape generator can generate only a cyclic waveform like rectangular waves or triangular waves. The rise/fall times of a pulse are also uncontrollable. Therefore, it is difficult to generate a waveform similar to a radiation spurious pulse. The apparatus cited hereunder are commercially available as apparatus which output radiation spurious pulses (see Nonpatent Documents 1 and 2, for example). In either case, waveform is adjusted by a switch or dial encoder attached to the main frame of the apparatus.                BNC Model BL-2 FAST Tail Pulse Generator (see Nonpatent Document 1)        
Characteristics                The following parameters are adjustable by a dial encoder attached to the apparatus:        output cycle (1 kHz-50 MHz)        pulse amplitude (0-3.0V)        adjustment of rise time (adjustable to 3, 5, 10, 30, 100 and 250 ns)        adjustment of fall time (adjustable to 5, 10, 30, 100, 300 ns and 1, 3 and 10 μs)                    Rise/fall times are discretely variable only to the preset values in the parentheses.            Outputs from two poles are possible.            Outputs can be made only in regular cycles.            BNC Model DB-2 FAST Tail Pulse Generator (see Nonpatent Document 2)                        
Characteristics                In pulse output timing, there are repeated/random output modes and changeover can be made with a switch attached to the apparatus.        Rise time is adjustable to 100 ns-20 μm (eight steps).        
A specific circuit of a spurious pulse generator will be described with reference to FIG. 4. FIG. 4 is a specific circuit diagram of a conventional spurious pulse generator. The spurious pulse generator, as shown in FIG. 4, has integrating circuits at a plurality of stages for carrying out integrating operations about time and outputting a spurious pulse. In FIG. 4, two stages of integrating circuits 101, 102 are provided, the upstream integrating circuit being integrating circuit 101, and the downstream integrating circuit being integrating circuit 102. The upstream integrating circuit 101 includes an amplifier AMP1, a variable resistor R1 connected to an input side of amplifier AMP1, and a capacitance C1 connected to the input side of amplifier AMP1 and connected to ground potential. The downstream integrating circuit 102 includes an amplifier AMP2, a variable resistor R2 connected to an input side of amplifier AMP2, and a capacitance C2 connected to the input side of amplifier AMP2 and connected to ground potential. An output side of amplifier AMP1 of the integrating circuit 101 and the variable resistor R2 of the integrating circuit 102 are connected in series.
On the other hand, a differentiating circuit 103 and an inverting amplifier circuit 104 are provided downstream of the integrating circuit 102. The differentiating circuit 103 includes a capacitance C3 connected to a resistor Rs of the inverting amplifier circuit 104 to be described hereinafter, and a resistor R3 connected to the resistor Rs and connected to ground potential. An output side of amplifier AMP2 of the integrating circuit 102 and the capacitance C3 of the differentiating circuit 103 are connected in series. The inverting amplifier circuit 104 includes the resistor Rs, a variable resistor Rf connected to the resistor Rs, and an operational amplifier OP connected to the resistor Rs.
The integrating circuit 101 is constructed such that a voltage value VDAC converted from a digital value into an analog value by a DA converter (DAC: Digital to Analog Converter) is inputted to the amplifier AMP1 of the integrating circuit 101 when a switching element SW is OFF. The voltage at the output side of the operational amplifier OP and the output side of the variable resistor Rf of the inverting amplifier circuit 104 is set to VOUT. The voltage at the input side of the amplifier AMP1 of the integrating circuit 101 is set to V1. The voltage at the input side of the amplifier AMP2 of the integrating circuit 102 is set to V2. The voltage at the output side of the resistor Rs and capacitance C3 of the differentiating circuit 103 and at the input side of the resistor Rs of the inverting amplifier circuit 104 is set to V3.
The above voltage value VDAC is a voltage value for controlling a crest value which is a peak swing of the spurious pulse. In order to generate a spurious pulse having various crest values, the value of voltage value VDAC is changed at each ON/OFF changeover of the switching element SW. Ground potential is inputted when the switching element SW is ON.